Trench DRAM Cell with Vertical Device and Buried Word Lines

ABSTRACT

A DRAM array having trench capacitor cells of potentially 4F 2  surface area (F being the photolithographic minimum feature width), and a process for fabricating such an array. The array has a cross-point cell layout in which a memory cell is located at the intersection of each bit line and each word line. Each cell in the array has a vertical device such as a transistor, with the source, drain, and channel regions of the transistor being formed from epitaxially grown single crystal silicon. The vertical transistor is formed above the trench capacitor.

RELATED PATENT DATA

The present application is a divisional of U.S. patent application Ser.No. 12/357,196 which was filed on Jan. 21, 2009, which is a continuationof application Ser. No. 10/962,657, filed Oct. 13, 2004, now U.S. Pat.No. 7,488,641, which is a divisional of application Ser. No. 10/640,387,filed Aug. 14, 2003, now U.S. Pat. No. 6,946,700, which is acontinuation application of application Ser. No. 10/152,842, filed May23, 2002, now U.S. Pat. No. 6,624,033, which is a continuationapplication of application Ser. No. 09/405,091, filed Sep. 27, 1999, nowU.S. Pat. No. 6,395,597, which is a divisional application ofapplication Ser. No. 09/204,072, filed Dec. 3, 1998, now U.S. Pat. No.5,977,579. The entirety of each of these applications is herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to an improved semiconductorstructure for high density device arrays, and in particular to a trenchDRAM cell array, and to a process for its formation.

BACKGROUND OF THE INVENTION

There are two major types of random-access memory cells, dynamic andstatic. Dynamic random-access memories (DRAMs) can be programmed tostore a voltage which represents one of two binary values, but requireperiodic reprogramming or “refreshing” to maintain this voltage for morethan very short time periods. Static random-access memories are so namedbecause they do not require periodic refreshing.

DRAM memory circuits are manufactured by replicating millions ofidentical circuit elements, known as DRAM cells, on a singlesemiconductor wafer. Each DRAM cell is an addressable location that canstore one bit (binary digit) of data. In its most common form, a DRAMcell consists of two circuit components: a field effect transistor (FET)and a capacitor.

FIG. 1 illustrates a portion of a DRAM memory circuit containing twoneighboring DRAM cells 42. For each cell, the capacitor 44 has twoconnections, located on opposite sides of the capacitor 44. The firstconnection is to a reference voltage, which is typically one half of theinternal operating voltage (the voltage corresponding to a logical “1”signal) of the circuit. The second connection is to the drain of the FET46. The gate of the FET 46 is connected to the word line 48, and thesource of the FET is connected to the bit line 50. This connectionenables the word line 48 to control access to the capacitor 44 byallowing or preventing a signal (a logical “0” or a logical “1”) on thebit line 50 to be written to or read from the capacitor 44.

The body of the FET 46 is connected to the body line 76, which is usedto apply a fixed potential to the body. In a present day conventionalbulk silicon DRAM, this connection is provided directly to the siliconbulk in which the array devices are formed. However, in SOI or otheroxide isolated devices, a separate means of body connection is needed tomaintain the body potential. Body lines are used to avoid floating bodythreshold voltage instabilities that occur when FETs are used onsilicon-on-insulator (SOI) substrates. These threshold voltageinstabilities occur because the body of the FET does not have a fixedpotential. Threshold voltage is a function of the potential differencebetween the source and the body of a FET, so if the body does not have afixed potential, then the threshold voltage will be unstable. Becausecontrol of the threshold voltage is especially critical in DRAM cells, abody line may be used to provide the body of the FET with a fixedpotential so that the threshold voltage of the FET may thereby bestabilized.

The manufacturing of a DRAM cell includes the fabrication of atransistor, a capacitor, and three contacts: one each to the bit line,the word line, and the reference voltage. DRAM manufacturing is a highlycompetitive business. There is continuous pressure to decrease the sizeof individual cells and to increase memory cell density to allow morememory to be squeezed onto a single memory chip, especially fordensities greater than 256 Megabits. Limitations on cell size reductioninclude the passage of both active and passive word lines through thecell, the size of the cell capacitor, and the compatibility of arraydevices with non-array devices.

Conventional folded bit line cells of the 256 Mbit generation withplanar devices have a size of at least 8F², where F is the minimumlithographic feature size. If a folded bit line is not used, the cellmay be reduced to 6 or 7 F². To achieve a smaller size, vertical devicesmust be used. Cell sizes of 4F² may be achieved by using verticaltransistors stacked either below or above the cell capacitors, as in the“cross-point cell” of W. F. Richardson et al., “A Trench TransistorCross-Point DRAM Cell,” IEDM Technical Digest, pp. 714-17 (1985). Knowncross-point cells, which have a memory cell located at the intersectionof each bit line and each word line, are expensive and difficult tofabricate because the structure of the array devices is typicallyincompatible with that of non-array devices. Other known vertical cellDRAMs using stacked capacitors have integration problems due to theextreme topography of the capacitors.

There is needed, therefore, a DRAM cell having an area of 4F² or smallerthat achieves high array density while maintaining structuralcommonality between array and peripheral (non-array) features. Alsoneeded is a simple method of fabricating a trench DRAM cell thatmaximizes common process steps during the formation of array andperipheral devices.

SUMMARY OF THE INVENTION

The present invention provides a DRAM cell array having a cell area of4F² or smaller which comprises an array of vertical transistors locatedover an array of trench capacitors. The trench capacitor for each cellis located beneath and to one side of the vertical transistor, therebydecreasing the cell area while maintaining compatibility of the verticaltransistors with peripheral devices. Also provided is a simplifiedprocess for fabricating the DRAM cell array which may share commonprocess steps with peripheral device formation so as to minimize thefabrication cost of the array.

Additional advantages and features of the present invention will beapparent from the following detailed description and drawings whichillustrate preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a known DRAM cell.

FIG. 2 is a perspective view of the memory array of the presentinvention.

FIG. 3 is a cross-sectional view of a semiconductor wafer undergoing theprocess of a preferred embodiment.

FIG. 4 shows the wafer of FIG. 3 at a processing step subsequent to thatshown in FIG. 3.

FIG. 5 shows the wafer of FIG. 3 at a processing step subsequent to thatshown in FIG. 4.

FIG. 6 shows the wafer of FIG. 3 at a processing step subsequent to thatshown in FIG. 5.

FIG. 7 shows the wafer of FIG. 3 at a processing step subsequent to thatshown in FIG. 6.

FIG. 8 shows the wafer of FIG. 3 at a processing step subsequent to thatshown in FIG. 7.

FIG. 9 shows the wafer of FIG. 3 at a processing step subsequent to thatshown in FIG. 8.

FIG. 10 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 9.

FIG. 11 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 10.

FIG. 12 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 11.

FIG. 13 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 12.

FIG. 14 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 13.

FIG. 15 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 14.

FIG. 16 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 15.

FIG. 17 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 16.

FIG. 18 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 17.

FIG. 19 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 18.

FIG. 20 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 19.

FIG. 21 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 20.

FIG. 22 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 21.

FIG. 23 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 22.

FIG. 24 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 23.

FIG. 25 shows the wafer of FIG. 3 at a processing step subsequent tothat shown in FIG. 24.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized, and thatstructural, logical and electrical changes may be made without departingfrom the spirit and scope of the present invention.

The terms “wafer” and “substrate” are to be understood as includingsilicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology,doped and undoped semiconductors, epitaxial layers of silicon supportedby a base semiconductor foundation, and other semiconductor structures.Furthermore, when reference is made to a “wafer” or “substrate” in thefollowing description, previous process steps may have been utilized toform regions or junctions in the base semiconductor structure orfoundation. In addition, the semiconductor need not be silicon-based,but could be based on silicon-germanium, germanium, or gallium arsenide.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims.

Referring now to the drawings, where like elements are designated bylike reference numerals, an embodiment of the device array 40 of thepresent invention is shown in FIG. 2. The device array 40 is comprisedof a plurality of trench DRAM cells 42 formed on a substrate 60, wherethe DRAM cells 42 are separated from each other by oxide isolationlayers 62. Each DRAM cell 42 comprises two components, a verticaltransistor 46, and a trench capacitor 44 located beneath the transistor46.

The transistor 46 forms a vertical stack of three doped silicon layersresting on top of the isolation layer 62. An exemplary n-channel device,as illustrated in FIG. 2, would be formed using a substrate 60 of afirst conductivity type, e.g., p+, a drain 70 of a second conductivitytype (n+), a lightly-doped body region 72 of a first conductivity type(p−), and a source 74 of a second conductivity type (n+). If a p-channeldevice were desired, the doping types and levels of these elements wouldbe adjusted as is known in the art. The capacitor 44 comprises apolysilicon electrode 80, which for exemplary purposes is of a secondconductivity type (n+), and a dielectric 82, which may be any suitabledielectric material such as oxide, ON (oxide-nitride), or ONO(oxide-nitride-oxide). The region of the substrate 60 underlying theelectrode 80 acts as a capacitor plate.

The transistor 46 is a MOSFET (metal-oxide-semiconductor FET) devicehaving four contacts to other portions of the cell 42 or array 40.First, the drain 70 of the transistor 46 is in contact with thecapacitor electrode 80. Second, a conductive bit line 50 formed ofpolysilicon doped to a second conductivity type (n+) is formed so thatit contacts the source 74 of each transistor 46 of a particular columnin the array 40. Third, an active word line 48 of a conductive materialsuch as doped polysilicon of a second conductivity type (n+) is formedto act as the gate of each transistor 46, and to electrically connectall of the cells 42 of a given row in the array 40. A thin oxide layer132 is present between the word line 48 and the body 72 of eachtransistor 46. Fourth, a body line 76 of a conductive material such asdoped polysilicon of a first conductivity type (p+) is formed to contactthe body 72 of each transistor 46 in a given row. The presence of thebody line 76 serves to avoid floating body threshold voltageinstabilities.

The device array 40 is manufactured through a process described asfollowing, and illustrated by FIGS. 3 through 25. For exemplarypurposes, dimensions are suggested which are suitable for 0.2 microncritical dimension technology, and it should be understood thatdimensions should be scaled accordingly for other critical dimensionsizes. First, a substrate 60, which may be any of the types of substratedescribed above, is selected as the base for the device array 40. Forexemplary purposes, the substrate 60 will be described as a siliconsubstrate, and the following process should be modified as appropriateand as known in the art if a non-silicon substrate is used. Thesubstrate 60 may be doped or undoped, but a p+ type doped wafer ispreferred. If PMOS devices are to be formed, photolithography is used todefine areas where n-wells (not shown) are implanted. The level ofdoping in the n-wells may vary but should be of comparable or greaterstrength than the doping level of the substrate 60.

As shown in FIG. 3, the first step in the process is to form the devicelayers 100, 102, 104. The device layers 100, 102, 104 are formed ofdoped epitaxial silicon by known methods of epitaxial growth, such asvapor phase, liquid phase, or solid phase epitaxy. If a siliconsubstrate 60 is used, then vapor phase epitaxy is preferred, and if aGroup III-V compound substrate, e.g., gallium arsenide or indiumphosphate, is used, liquid phase epitaxy is preferred. For the formationof the device array 40 of the present embodiment, the first device layer100 should be a doped silicon layer of a second conductivity type (n+)approximately 0.4 microns thick, the second device layer 102 should be alightly-doped silicon layer of a first conductivity type (p−)approximately 0.35 microns thick, and the third device layer 104 shouldbe a doped silicon layer of a second conductivity type (n+)approximately 0.2 microns thick.

Next, as shown in FIG. 4, an oxide pad 106 approximately 10 nm thick,and a first nitride pad 108 approximately 100 nm thick are formed on topof the third device layer 104 by chemical vapor deposition (CVD) orother suitable means. A photoresist and mask are then applied over thefirst nitride pad 108, and photolithographic techniques are used todefine a set of parallel columns on the array surface. A directionaletching process such as plasma etching or reactive ion etching (RIE) isused to etch through the pad layers 106, 108 and the device layers 100,102, 104 and into the substrate 60 to form a first set of trenches 110,as depicted in FIG. 5. The trenches 110 should be approximately 1.05microns deep.

After removal of the resist, the first set of trenches 110 is filledwith silicon oxide by CVD or other suitable process to form a first setof silicon oxide bars 112, as shown in FIG. 6. The device array 40 isthen planarized by any suitable means, such as chemical-mechanicalpolishing (CMP), stopping on the first nitride pad 108. A second nitridepad 114 is then deposited, preferably by CVD, to a thickness of about 60to 100 nm. The device array 40 now appears as shown in FIG. 6.

FIG. 7 illustrates the next step in the process, in which a resist andmask (not shown) are applied, and photolithography is used to define asecond set of trenches 116 orthogonal to the first set of silicon oxidebars 112. The nitride pads 108, 114, the oxide pad 106, and the exposeddevice layers 100, 102, 104 are etched out by a directional etchingprocess such as RIE to define the second set of trenches 116. Etching iscontinued down to the level of the substrate 60, and the second set oftrenches 116 should be approximately 0.95 microns deep. The resist isthen removed. As can be seen, the second set of trenches 116 is definedby a set of device islands 118, which will be transformed intoindividual DRAM cells by the fabrication process described herein.

As shown in FIG. 8, a first nitride film 120 is now formed on the sidesof the second set of trenches 116 by depositing a layer of CVD nitrideand directionally etching to remove excess nitride from horizontalsurfaces. The first nitride film 120, which is about 10 nm thick, actsas an oxidation and etching barrier during subsequent steps of thefabrication process. Isotropic etching such as RIE is then performed todeepen the second set of trenches 116 an additional 0.1 microns andundercut the device island resulting in the structure shown in FIG. 9.

Thermal oxidation is then performed to create an isolation layer 62under and between the device islands 118, as depicted by FIG. 10. Thesubstrate 60 is thermally oxidized by a suitable process as known in theart, such as by heating the wafer in a standard silicon processingfurnace at a temperature of approximately 900 to 1100 degrees Celsius ina wet ambient. The oxidation time is selected to produce an isolationlayer 62, at least approximately 0.1 microns thick under the deviceislands 118.

FIG. 11 shows the next step in the process, in which an anisotropic etchsuch as RIE is performed to deepen the second set of trenches 116through the isolation layer 62 and into the substrate 60 to the depthdesired for the trench capacitors.

As illustrated in FIG. 12, a capacitor dielectric layer 82 is now formedinside the second set of trenches 116 on the sides of the device islands118 and the bottom of the trenches 116. The dielectric layer 82 may beoxide, ON, or ONO, and is preferably formed by CVD and/or thermaloxidation. Next, the second set of trenches 116 are filled with apolysilicon layer 80 of a second conductivity type (n+) by CVD or othersuitable means, as shown in FIG. 13.

The polysilicon layer 80 is then etched back to a level approximately 1micron below the second nitride pad 114, as shown in FIG. 14. Thecapacitor dielectric on exposed sidewalls 82 is then removed byisotropic etching. The second set of trenches 116 are then re-filledwith polysilicon of a second conductivity type (n+) by CVD. Thepolysilicon is then etched back to a level approximately 0.55 micronsbelow the second nitride pad 114 to form a capacitor electrode 80,depicted in FIG. 15.

FIG. 16 illustrates the next step of the process in which the exposedsegments of oxide bars 112 are etched back by ˜0.4 μm followed by thedeposition of a nitride film 122 on the sides of the second set oftrenches 116. The film 122, which is about 10 nm thick, is formed bydepositing a layer of CVD nitride and directionally etching to removeexcess nitride from horizontal surfaces. The second nitride film 122acts as an oxidation barrier during the next step of the process.

Thermal oxidation of the capacitor electrode 80 is now performed bymethods known in the art to create a first oxide layer 124 approximately100 nm thick on top of the electrode 80 in the second set of trenches116. The second nitride film 122 is then stripped from the sides of thedevice islands 118 and remaining segments of oxide 112 in trenches 116,preferably by isotropic etching with a nitride etchant such asphosphoric acid, to form the structure shown in FIG. 17.

FIG. 18 depicts the following step of the process where polysilicon of afirst conductivity type (p+) is deposited by CVD or other suitable meansin the second set of trenches 116 to a thickness of approximately 70 nm.A directional etch such as RIE is performed so that no polysiliconremains on the horizontal surfaces of the array 40, and the etch iscontinued to recess the top of the polysilicon to at least 0.2 micronsbelow the bottom of the oxide pad 106. The resultant first and secondbody lines 76, 78 are shown in FIG. 18. A conformal film 126 of nitrideor other suitable material is now formed over the first and second bodylines 76, 78, as shown in FIG. 19. The conformal film 126 isapproximately 10 nm thick, and is formed by CVD or other suitablemethods.

As illustrated in FIG. 20, a photoresist and mask are applied, andphotolithography is used to define a third set of trenches 128 insidethe second set of trenches 116. The exposed conformal film 126 is etchedoff by an isotropic etch, and then a directional etch is performed toremove the second body line 78. Directional etching is continued toremove the exposed first oxide layer 124 and the exposed electrode 80 toa depth below the first device layer 100.

The resist is stripped, and the third set of trenches is filled withsilicon oxide by CVD or known methods, as shown in FIG. 21. If desired,the device array 40 may be planarized by CMP or other means at thispoint. The silicon oxide is then etched back to form a second oxidelayer 130 at a level approximately 0.6 to 0.7 microns below the level ofthe oxide pad 106.

FIG. 22 depicts the next step, in which a thin gate oxide layer 132 isformed by thermal oxidation of the exposed side of the device islands118. Next, polysilicon of a second conductivity type (n+) is depositedby CVD or other suitable means in the third set of trenches 128 to athickness of approximately 70 nm. A directional etch such as RIE isperformed so that no polysilicon remains on the horizontal surfaces ofthe array 40. A resist and mask (not shown) are then applied, and aselective etch is performed to remove excess polysilicon on the bodyline 76 side of the third set of trenches 128. The resultant word line48 is shown in FIG. 22.

The resist is stripped, and the third set of trenches 128 are filledwith silicon oxide by CVD or other suitable means to form a second setof silicon oxide bars 134, as shown in FIG. 23. The device array 40 isthen planarized by any suitable means, such as CMP, stopping on thesecond nitride pad 114. FIG. 24 illustrates the next step in theprocess, in which a dip etch is performed to remove the nitride pads108, 114 and the oxide pad 106 from the device islands 118.

As shown in FIG. 25, the next step is to form a conductive bit line 50over the device array 40 so that it contacts the source 74 of eachtransistor 46 of a particular row in the array 40. The bit line 50 isformed of doped polysilicon of a second conductivity type (n+), and isdeposited by means such as CVD. After deposition, the polysilicon ispatterned by photolithography and subsequent etching to form a bit line50 as shown in FIG. 25 and in FIG. 2. Conventional processing methodsmay then be used to form contacts and wiring to connect the device arrayto peripheral circuits, and to form other connections. For example, theentire surface may then be covered with a passivation layer of, e.g.,silicon dioxide, BSG, PSG, or BPSG, which is CMP planarized and etchedto provide contact holes which may then be metallized. Conventionalmultiple layers of conductors and insulators may also be used tointerconnect the structures.

As can be seen by the embodiments described herein, the presentinvention encompasses a trench DRAM cell having an area of 4F² orsmaller that comprises a vertical transistor located over a trenchcapacitor. As may be readily appreciated by persons skilled in the art,decreasing the size of the DRAM cell while maintaining common processsteps with peripheral devices decreases fabrication costs whileincreasing array density. As a result, a high density and highperformance array is produced by a simplified fabrication process.

The above description and drawings illustrate preferred embodimentswhich achieve the objects, features and advantages of the presentinvention. It is not intended that the present invention be limited tothe illustrated embodiments. Any modification of the present inventionwhich comes within the spirit and scope of the following claims shouldbe considered part of the present invention.

1. A method of forming a DRAM array comprising: providing a siliconsubstrate; forming a first silicon layer of a first conductivity type onthe substrate; forming a second silicon layer of a second conductivitytype on the first silicon layer; forming a third silicon layer of thefirst conductivity type on the second silicon layer; defining a firstset of trenches in the substrate an extending through the first, secondand third silicon layers; defining a second set of trenches in thesubstrate which extend through the first, second and third siliconlayers in a direction orthogonal to the first set of trenches to formvertical stacks of the first second and third silicon layers; isotropicetching to deepen the second set of trenches wherein the isotropicetching undercuts the vertical stacks; creating an isolation layer underand between the vertical stacks; forming word lines in contact with afirst side of the vertical stacks; and forming bit lines on top of thethird silicon layer of the vertical stacks.
 2. The method of claim 1wherein the word lines and vertical stacks form vertical transistorsspanning a surface area over the substrate of no greater than 4F², whereF is the minimum lithographic feature size.
 3. The method of claim 1wherein the creating the isolation layer comprises thermal oxidation ofthe substrate.
 4. The method of claim 1 wherein prior to the isotropicetching, a nitride film is formed over sides of the second set oftrenches.
 5. The method of claim 1 further comprising electricallyconnecting the vertical stacks with individual capacitor structures. 6.The method of claim 5 wherein the capacitor structures are trenchcapacitor structures.
 7. The method of claim 5 wherein the trenchcapacitor structures are disposed within the second set of trenches. 8.The method of claim 5 wherein the substrate serves as one plate of thetrench capacitor structures.